From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH 01/18] flag parameters: helper function Date: Mon, 5 May 2008 19:56:38 -0700 Message-ID: <20080505195638.4c4bdc40.akpm@linux-foundation.org> References: <200805050342.m453gkY4029814@devserv.devel.redhat.com> <20080505185103.b9299541.akpm@linux-foundation.org> <481FC4E6.5060206@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, davidel@xmailserver.org, mtk.manpages@gmail.com, torvalds@linux-foundation.org To: Ulrich Drepper Return-path: Received: from smtp1.linux-foundation.org ([140.211.169.13]:48470 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752530AbYEFC47 (ORCPT ); Mon, 5 May 2008 22:56:59 -0400 In-Reply-To: <481FC4E6.5060206@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On Mon, 05 May 2008 19:39:34 -0700 Ulrich Drepper wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Andrew Morton wrote: > > hm, that looks expensive. The compiler will need to generate a deref of m > > and rf multiple times around the loop. Copying them into locals does > > improve that a lot. > > There really is no problem. The value is in L1d when it is reused. > This is the generated code (%rdi is m): > > f: 85 17 test %edx,(%rdi) > 11: 74 0b je 1e > 13: 8b 47 04 mov 0x4(%rdi),%eax > 16: 09 01 or %eax,(%rcx) the deref of %rcx can be avoided. > 18: 8b 07 mov (%rdi),%eax > 1a: f7 d0 not %eax > 1c: 21 c2 and %eax,%edx > > At address 18 the load will be satisfied from L1d. If you would want to > cache the value at address f you'd have to create one more instruction. > > This really is the best code sequence. The compiler could have chosen > to move the value into a register because the array is const. But it > didn't. > > > > Also: sorry, but ugh-at-the-naming. We don't *gain* anything from having > > idenitifers called f, of, m, n and rf. And we lose quite a lot in > > readability and understandability. It would be much nicer to invest a > > little bit more typing-time here, IMO. > > That's Davide's code and I didn't change it because it doesn't really > matter. This is a trivial function which doesn't need more than 10 > seconds to be understood. If you insist I'll rename the variables and > elements but I consider this just busy work. Well if the objective is saving work then why write any code at all?